1. Field of the Invention
The present invention relates to a projection-type image display apparatus that displays a color image with one light valve as a light modulating member.
2. Description of Related Art
A liquid crystal projector now part of the mainstream in the market of large-screen displays uses a light source lamp, a focusing lens and a projection lens to magnify and form an image of a liquid crystal panel (a light valve) onto a screen. Currently commercialized systems can be classified roughly into a three-plate system and a single-plate system.
In the former system of the three-plate liquid crystal projector, after a light beam from a white light source is separated into light beams of three primary colors of red, green and blue by a color separation optical system, these light beams are modulated by three monochrome liquid crystal panels so as to form images of the three primary colors. Thereafter, these images are combined by a color combination optical system so as to be projected onto a screen by one projection lens. Since the entire spectrum of the white light from the light source can be utilized, this system has a high efficiency of light utilization. However, because of the necessity of the three liquid crystal panels, the color separation optical system, the color combination optical system and a convergence adjusting mechanism between the liquid crystal panels, this system is relatively expensive.
On the other hand, a conventional single-plate system liquid crystal projector is compact and inexpensive because an image formed on a liquid crystal panel having a mosaic color filter simply is magnified and projected onto a screen. However, since this system obtains light with a desired color by absorbing light with a unwanted color out of white light from the light source by means of the color filter as a color selection member, only one-third or less of the white light that has entered the liquid crystal panel is transmitted (or reflected). Accordingly, the efficiency of light utilization is low and high-brightness images cannot be obtained easily. When the light source is brightened, the brightness of the displayed image can be improved. However, there remain problems of heat generation and light resistance owing to light absorption by the color filter, making it very difficult to increase the brightness.
In recent years, as a way to eliminate light loss owing to the color filter in this single-plate system, a new configuration in which the efficiency of light utilization is raised by using dichroic mirrors and a microlens array instead of the color filter has been suggested and also commercialized.
A conventional single-plate projection-type image display apparatus, which improves the efficiency of light utilization using the dichroic mirrors and the microlens array, will now be described. FIG. 7 shows a schematic configuration thereof, and FIG. 8 shows a detailed cross-section of a light valve of the projection-type image display apparatus shown in FIG. 7.
A projection-type image display apparatus 900 has a light source portion 901, an illuminating device 903, a color separation optical system 907, a transmission-type light valve 902 and a projection lens 908. A white light beam from the light source portion 901 irradiates an effective region of the light valve 902 by means of the illuminating device 903. The color separation optical system 907 includes a red-reflecting dichroic mirror 904, a green-reflecting dichroic mirror 905 and a total reflection mirror 906 that are arranged obliquely. The white light beam that has passed through the illuminating device 903 enters the color separation optical system 907, thereby being separated horizontally into three light beams of primary colors of red, green and blue, so as to enter the light valve 902. The transmission-type light valve 902 has pixels that can modulate the incident light beams of the respective colors independently by an input signal corresponding to each of the red, green and blue light beams. These pixels are arranged horizontally in one element.
The white light beam emitted from the light source portion 901 is led to the color separation optical system 907 by the illuminating device 903. A red light beam in the incident light is reflected by the red-reflecting dichroic mirror 904 placed obliquely with respect to the incident light so as to travel along an optical axis 909. A green light beam in the light transmitted by the red-reflecting dichroic mirror 904 is reflected by the green-reflecting dichroic mirror 905 placed obliquely with respect to the incident light so as to travel along an optical axis 910. A blue light beam transmitted by the green-reflecting dichroic mirror 905 enters the reflection mirror 906, and is then reflected so as to travel along an optical axis 911. The red light beam on the optical axis 909, the green light beam on the optical axis 910 and the blue light beam on the optical axis 911 pass through a condenser lens 912 and reach the transmission-type light valve 902.
As shown in FIG. 8, an entrance-side polarizing plate 913 as a polarizer is provided on the side of an entrance surface of the transmission-type light valve 902, and only the light beam having a predetermined polarization direction in the incident light is transmitted by this polarizing plate 913. The transmitted light enters a microlens array 918 including a group of microlenses 917 with their longitudinal direction being in a vertical direction. The horizontal width of the microlens 917 corresponds to the total horizontal widths of a pixel aperture for red 914, a pixel aperture for green 915 and a pixel aperture for blue 916. The red light beam that has traveled along the optical axis 909 and entered the microlens 917 obliquely at an incident angle of 01 is focused on the pixel aperture for red 914. The green light beam that has traveled along the optical axis 910 and whose chief ray entered the microlens 917 at a right angle is focused on the pixel aperture for green 915. The blue light beam that has traveled along the optical axis 911 and entered the microlens 917 obliquely from the direction opposite to the red light at an incident angle of xcex81 is focused on the pixel aperture for blue 916.
The light beam of each color that has passed through the pixel aperture for each color enters an exit-side polarizing plate 919 provided on an exit surface of the transmission-type light valve 902. The exit-side polarizing plate 919 has a polarization axis arranged orthogonal to the polarization axis of the entrance-side polarizing plate 913. Since a light beam that has entered a pixel aperture to be displayed as white is emitted with its polarization direction being rotated by about 90xc2x0 in a liquid crystal layer, it is transmitted by the exit-side polarizing plate 919 and reaches the projection lens 908. Since a light beam that has entered a pixel aperture to be displayed as black is emitted without being subjected to the rotation of its polarization direction in the liquid crystal layer, it is absorbed by the exit-side polarizing plate 919 and does not reach the projection lens 908. The transmission-type light valve 902 rotates the polarization direction of the incident light at every pixel so as to display an image.
In the single-plate projection-type image display apparatus with the new configuration in which the efficiency of light utilization is raised as described above, it is possible to achieve a high efficiency of light utilization close to that in the three-plate system without wasting the light from the light source.
However, in this configuration, a bright lens whose f-number is smaller than 1/(2 sin (xcex82+xcex83)) is required as the projection lens 908, where a half-angle of a cone of rays converging from the microlens 917 toward the pixel aperture is expressed by xcex82 and an incident angle at which the chief ray of the red light or the blue light enters the pixel aperture is expressed by xcex83. An actual f-number is 1.0 to 1.5.
Accordingly, even when the single-plate system is adopted so as to use one display device, the size and the cost of the projection lens increase in practice. Thus, its advantage over the three-plate system is not readily apparent.
Furthermore, since a light beam of each color from the light source is led to the pixel of a corresponding color, the resolution of an image display panel (the transmission-type light valve 902) has to be three times as high as a necessary resolution in order to achieve high resolution. This increases the cost of the image display panel, and also lowers transmittance when the transmission-type light valve is used as the image display panel. Moreover, when the resolution of the image display panel is low, or when an image is magnified considerably, colors of red, green and blue look separate, causing image quality deterioration such as convergence dislocation.
In response to the above problems, an image display apparatus is suggested in JP 4(1992)-316296 A. FIG. 9 shows a schematic configuration of this image display apparatus.
A white light beam emitted from a light source portion 920 is led to a color separation optical system 921. As shown in FIG. 10, the color separation optical system 921 includes dichroic mirrors 921a and 921b and two reflection mirrors 921c and 921d. The dichroic mirror 921a reflects blue light and transmits green light and red light. The dichroic mirror 921b reflects red light and transmits green light and blue light. These dichroic mirrors 921a and 921b are crossed. A blue light beam 932 out of a white light beam 931 from the light source portion 920 is reflected by the dichroic mirror 921a, reflected by the reflection mirror 921d and passes through an aperture 922b of a field stop 922. A red light beam 933 is reflected by the dichroic mirror 921b, reflected by the reflection mirror 921c and passes through an aperture 922r of the field stop 922. A green light beam 934 is transmitted by both the dichroic mirrors 921a and 921b and passes through an aperture 922g of the field stop 922. The apertures 922r, 922g and 922b of the field stop 922 are formed like a belt (a rectangle), and the light beams of red, green and blue are emitted adjacent to each other from these apertures.
The belt-like light beams of respective colors emitted from the field aperture 922 pass through a scanning optical system 924, then illuminate different regions of a single transmission-type light valve (a display panel) 923 in a belt-like manner. With an effect of a rotating prism 924a constituting the scanning optical system 924, the belt-like light beams of red, green and blue scan the light valve 923 from the bottom to the top. When a belt-like illuminated region of one of the light beams goes beyond the uppermost end of an effective region of the light valve 923, the belt-like illuminated region of this light beam appears at the lowermost end of the effective region of the light valve 923 again. In this manner, the light beams of red, green and blue can continuously scan over the entire effective region of the light valve 923.
A light beam illuminating each row on the light valve 923 varies moment by moment, and a light valve driving device (not shown in this figure) drives each pixel by an information signal according to the color of the light beam that is illuminated. This means that each row of the light valve 923 is driven three times at every field of a video signal to be displayed. A driving signal inputted to each row is a color signal corresponding to the light beam illuminating this row among signals of the image to be displayed. The light beams of these colors that have been modulated by the light valve 923 are magnified and projected onto a screen (not shown in this figure) by a projection lens 925.
With the above configuration, the light beam from the white light source is separated into light beams of three primary colors, so that the light from the light source can be used with substantially no loss and the efficiency of light utilization can be increased. Also, since each of the pixels on the light valve displays red, green and blue sequentially, colors do not look separate unlike the above-described single-plate projection-type image display apparatus (see FIGS. 7 and 8). Therefore, it is possible to provide a high quality image.
However, in the above configuration, the light beams of these colors from the field stop 922 are not focused when transmitted by the rotating prism 924a. Since the size (the radius of gyration) of the rotating prism 924a has to be in accordance with a region illuminated by the light beam emitted from the field stop 922, the rotating prism 924a becomes large and heavy. This has made it difficult to reduce the size and weight of the apparatus.
Furthermore, a powerful motor for rotating the rotating prism 924a becomes necessary, causing an increase in the size and cost of the apparatus.
In addition, the single-plate projection-type image display apparatus and the three-plate projection-type image display apparatus are both placed on a desk or the like and used for a presentation or viewing movies. Accordingly, in order not to get in the way of viewing, most of them are thin horizontal types in which component parts are laid out horizontally so as to reduce its height, while vertical types in which component parts are laid out vertically are hardly seen.
It is an object of the present invention to solve the above-described problems of the conventional projection-type image display apparatus and to provide a projection-type image display apparatus that is provided with a scanning optical system for scanning an illuminated portion (a light valve) sequentially with light beams of individual colors, has a high efficiency of light utilization and component parts laid out horizontally, and is small and thin.
In order to achieve the above-mentioned object, the present invention has the following configurations.
A projection-type image display apparatus of the present invention includes a light source portion for emitting respective light beams of red, green and blue, a first optical system that the respective light beams from the light source portion enter, a rotating polygon mirror that the respective light beams having left the first optical system enter and that makes the respective light beams perform a scanning while reflecting the respective light beams, a second optical system for leading the respective light beams from the rotating polygon mirror to an illumination position, an image display panel that is arranged at the illumination position and provided with many pixels for modulating an incident light according to a color signal of red, green or blue, an image display panel driving circuit for driving each of the pixels of the image display panel by a signal corresponding to a color of light entering this pixel, and a projection optical system for magnifying and projecting an image of the image display panel. Belt-like regions illuminated by the respective light beams are formed substantially in parallel with each other on the image display panel and moved continuously by the scanning, thereby displaying a color image. The second optical system includes at least one lens and an optical element for rotating the respective light beams from the rotating polygon mirror substantially by 90xc2x0 and directing them to the illumination position.
This makes it possible to display the color image using a light valve having neither a color filter nor a pixel exclusively for a light beam of each color, thereby achieving a higher efficiency of light utilization and a higher resolution display. Also, since the second optical system rotates the light beam from the rotating polygon mirror by 90xc2x0 and leads it to the image display panel, it is possible to reduce the thickness of the apparatus in a direction of a rotation axis of the rotating polygon mirror. Thus, a small and thin image display apparatus can be achieved using a widely-used light valve.
In the projection-type image display apparatus described above, it is preferable that the optical element of the second optical system includes at least two total reflection mirrors arranged obliquely. This makes it possible to provide a small and thin projection-type image display apparatus with a simple configuration in a low cost manner.
Also, in the projection-type image display apparatus described above, the light source portion may include a light source for emitting a white light beam including red, green and blue light beams and a color separation optical system for separating the white light beam into the red, green and blue light beams. By using a white light source and obtaining the light beams of red, green and blue by the color separation optical system, the light from the light source can be utilized still more efficiently.
Furthermore, in the projection-type image display apparatus described above, it is preferable that the light source portion includes an integrator optical system. This makes it possible to ensure uniformity of illumination in a direction orthogonal to a scanning direction of the image display panel.
Moreover, in the projection-type image display apparatus described above, it is preferable that the lens of the second optical system includes at least one fxcex8 lens. This makes it possible to move (scan) an illuminated region easily on the image display panel.
In addition, in the projection-type image display apparatus described above, the image display panel may be a transmission-type light valve. Alternatively, the image display panel may be a reflection-type light valve.